1. Field of the Invention
This invention relates to the electrolytic production of a metal from metallic oxide using a salt bath. More particularly, the invention relates to continuously producing a metallic halide using a unique anode under compression while depositing the metal at the cathode. More particularly, alumina is deposited by electrolytic deposition from the alumina oxide using energy-saving low electrical potentials.
Production of a metallic product by electrolysis may be illustrated by reference to the production of aluminum. The commercial production of aluminim is typically accomplished by the Hall-Heroult process. In this process, the purified source of alumina is dissolved in a molten all-fluoride salt solvent particularly consisting of cryolite, and then reduced electrolytically with a formed carbon anode according to the reaction: EQU 1/2Al.sub.2 O.sub.3 +3/4C+3e.sup.- .fwdarw.Al+3/4CO.sub.2 EQU 1/2Al.sub.2 O.sub.3 +3/2C+3e.sup.- .fwdarw.Al+3/2CO.
Four characteristics of the Hall-Heroult process include: (1) carbon dioxide being produced and a carbon anode being consumed at the rate of 0.33:1 pound of carbon per pound of aluminum produced, which results in a required continual movement of the formed carbon anode downwardly toward the cathode aluminum pool at the bottom of the cell to maintain constant spacing for uniform aluminum production and thermal balance in the cell; (2) the need to feed intermittently and evenly solid alumina in a limited concentration range to the open cell to maintain peak efficiency of operation and in order to avoid the anode defects; (3) severe corrosion of the cell materials due to high temperatures of 950.degree. to 1000.degree. C. in the fluoride bath resulting in low cell life and increased labor; and (4) cell power efficiency is limited to less than 50% since a carbon-anode-to-liquid-aluminum distance greater than one inch to reduce magnetic fields' undulation of the aluminum layer which would cause intermittent shorting and result in Faradaic losses due to a back reaction of aluminum droplets with carbon dioxide to produce alumina.
It is known that, in the Hall-Heroult cell reaction, the carbon of the anode contributes to the overall reaction of winning aluminim by decreasing the decomposition voltage. For example, the decomposition of A1.sub.2 O.sub.3 in cryolite on a platinum anode is about 2.2 volts, but on a carbon electrode the decomposition voltage is about 1.2 volts.
In the all-fluoride-containing bath, the alumina will dissolve in the cryolite-fluoride salt bath at a temperature of 950.degree. to 1000.degree. C. Bayer alumina is soluble in a cryolite-containing bath at a temperature of at least 900.degree. C. A fluoride-containing bath having a temperature below about 900.degree. C. will not readily solubilize ordinarily processed Bayer alumina and, therefore, the alumina, as a source of aluminum, cannot enter the reduction reaction nor is it possible for the aluminum to be deposited at the cathode.
2. Pertinent Art
Pertinent art is illustrated by the process described in U.S. Pat. No. 4,257,855 ('855), which discloses an electrolytic cell for the production of aluminum metal including a permanent hollow anode structure of corrosion-resistant material, with numerous perforations in the base of the structure, a packed bed of consumable carbon pieces supported by the base within the hollow space of the structure, a molten cryolite bath, means for adding fresh pieces of the consumable carbon to replenish the packed bed, and means for adjusting the depth of immersion of the packed bed within the molten cryolite bath so as to reduce voltage and energy requirements or increase the rate of aluminum production. The structure in this disclosure is the current carrying the anode and the carbon pieces are the supply of reducing material.
Pertinent art is further illustrated in International Publication No. WO87/00170, which discloses a process for the electrolytic reduction of aluminum from alumina using a carbon cathode disposed in a molten salt electrolyte solvent bath in which the alumina has been dissolved and which has a density less than the reduced molten aluminim. The steps of the process include continuously providing a particulate, free-flowing, high purity, and highly conductive carbon material to the molten bath to serve as the anode. The particulate carbon material has a density less than the molten bath. An electrical connection is placed in contact with the particulate carbon anode material and an electric current is applied. Reduced aluminum is collected at the cathode. The particulate carbon material is preferably formed from desulfurized petroleum coke which may be partially graphitized. The particulate material is required to have a lower density than the molten electrolytic bath such that it floats on the bath surface. It is also preferred that the material be relatively nonreactive with oxygen.
Pertinent art also includes U.S. Pat. No. 2,764,530 ('530), which discloses use of petroleum coke in an aluminum electrolytic process, the coke having an acceptable level of specific resistivity of about 0.0019 ohms per cubic inch. The patent, however, discloses the use of pitch with the petroleum coke and finds that use of 100 mesh ground coal without the pitch has unacceptably high specific resistance. The coal is ground treated carbon tetrachloride and acid, and mixed with pitch to achieve an acceptable level of resistivity.